1. Field of the Invention
The present invention relates generally to the field of internal combustion engines and more particularly, to internal combustion engines for motorcycles. Still more particularly, the present invention relates to the design of the crankcase thereof to provide oil return ducts of a specific configuration to improve engine performance and prevent oil adulteration.
2. Description of the Prior Art
In vehicle internal combustion engines of the prior art, for example, the engine described in U.S. Pat. No. 3,688,871, considerable power losses occur due to oil adulteration. The adulteration affects the performance of the crankshaft, the crank webs and the connecting rod heads in the crankcase and in the oil space respectively. The adulteration problem is particularly increased if the distance between the outer path of movement of the crank webs (hereinafter "the envelope of crank web travel") and the floor of the oil space is very small. To increase such distance is contrary to other criteria of engine design, i.e., maintaining a very low structural height. That criteria is especially important in the design of internal combustion engines for motorcycles.
The small clearance causes the rotating drive components of the engine to constantly dip into the oil flowing off from the floor part of the oil space into the oil sump. As a result, substantial volumes of oil are pulled up onto the rotating drive components, leading to relatively intense foaming within the crankcase. The foaming in turn causes a reduction in lubricating action leading to the reductuion in engine performance.
The aforementioned U.S. Letters Patent addresses this problem by providing ribs in the oil space to counteract a delay in oil discharge. The ribs should accelerate the oil supply and thus counteract a reduction in the permanent quality of oil in the sump and the tendency for oil temperature to rise. It has been found, however, that these desirable objects are not satisfactorily achieved because of the rib arrangement and their cooperation with the driving parts of the engine. A design which more satisfactorily achieves these objects and overcomes other deficiencies of prior art engine design would represent a significant advance in the art.